will 1200ma ac driver power 650ma 3w Led?
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Old 01-11-2014, 09:54 PM   #1
zimdoc
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will 1200ma ac driver power 650ma 3w Led?


After sending lots of time researching led and tonnes of useful posts in this forum I finally made the decision to purchase all the bits I needed. Just doing some final checks before diving in and realised that my ac driver 24-39v runs at 1200ma but LEDs are 650ma 😕?!

So my question is will I be able to use this driver or will the current blow the LEDs ?

Planning to wire these in 9x3w series
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Old 01-11-2014, 10:01 PM   #2
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Quote:
Originally Posted by zimdoc View Post
After sending lots of time researching led and tonnes of useful posts in this forum I finally made the decision to purchase all the bits I needed. Just doing some final checks before diving in and realised that my ac driver 24-39v runs at 1200ma but LEDs are 650ma 😕?!

So my question is will I be able to use this driver or will the current blow the LEDs ?

Planning to wire these in 9x3w series
If the driver's current can't be adjusted- then yes- it will let the "magic smoke" out of your leds.
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Old 01-11-2014, 10:35 PM   #3
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LEDs typically use a certain voltage depending on the color. According to ohms law your 3W LEDs at 650mA will take approximately 4.615 volts. 9 in series would require around 41.5 volts. They should be able to operate like this and the limit the current to around 650mA you could wire a 70 ohm resistor in parallel to the LEDs wired in series though the power rating for such a resistor would require 25 watts. You could also wire a resistor in series with the LEDs though I can't determine the correct value since the voltage of your LEDs will exceed your ac drivers output voltage

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Old 01-11-2014, 10:57 PM   #4
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Originally Posted by Salmon McCloud View Post
LEDs typically use a certain voltage depending on the color. According to ohms law your 3W LEDs at 650mA will take approximately 4.615 volts. 9 in series would require around 41.5 volts. They should be able to operate like this and the limit the current to around 650mA you could wire a 70 ohm resistor in parallel to the LEDs wired in series though the power rating for such a resistor would require 25 watts. You could also wire a resistor in series with the LEDs though I can't determine the correct value since the voltage of your LEDs will exceed your ac drivers output voltage

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Not true. LEDs don't operate by Ohm's Law. The driver will put out 1200 mAmps, with the voltage self-adjusting to whatever the load requires at that current, within limits. You can put the LEDs in two parallel strings of 9 LEDs in series, and each string will get 600 mAmps. That will work fine until a LED fails open circuit. At that time the full 1200 mAmps will go to the parallel string, probably electrocuting them. But, you can also put a fast blow 700 mAmp fuse in each of the two parallel strings, and then it won't go up in smoke.
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Old 01-11-2014, 11:04 PM   #5
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They are a diode. They take a certain voltage depending on the color. I was assuming the 3 watts is at max current or 650mA and that would be their typical voltage that they require. Current just determines the brightness of the LEDs. The higher the current the brighter. If you did want to purchase 9 more LEDs what hoppy suggests would work pretty well. As long as you have equal LEDs on both legs of the circuit you will have equal current on each leg. So you could just buy 7 more and have 8 on each leg. I would suggest not going lower than 6 on each leg of the circuit. Also definitely go with a fuse on each leg because replacing one led would be easier and cost efficient compared to the alternative.

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Old 01-11-2014, 11:19 PM   #6
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Thanks guys. Phew glad I checked.

Have just been browsing on no ebay links allowed Will the specs on this driver be suitable
escription:

Input Voltage: AC 85V~265V, 50Hz~60Hz
Output Voltage: DC 36V~75V 650MA
TC: 75
TA: 50 (MAX)
Waterproof Rating: IP67
Weight: 118g

Size:127* 26* 18mm


Salmon McCloud thanks for the idea resistor but not confident enough to fiddle
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Old 01-11-2014, 11:23 PM   #7
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Yeah that will work pretty perfectly. Though if 650mA is the max current I would recommend not running it at this current to extend the life of the LEDs

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Old 01-11-2014, 11:32 PM   #8
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Even easier- Buy a MeanWell LDD-600HW constant current driver and wire it in to your string of 9 leds. The driver will be keep the leds at a constant 600ma without the need for complicated circuitry, power robbing resistors ect... You may even be able to power it using the AC driver that you already have. The best part is they're cheap. Here's a link to MeanWell in the UK.http://www.meanwell.co.uk/led-power-.../ldd-600h.html
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Old 01-12-2014, 08:44 AM   #9
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Thanks

But I'm a little confused now? So my driver max current needs to be slightly than the max current for my LEDs say 700ma but should not exceed that or it will blow them?
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Old 01-12-2014, 01:30 PM   #10
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650mA is rated current not max current.

CREE XM's are rated at 700mA but can be run at 3000 with proper cooling. this turns a 3W piece into a 10W, so power is an important considering when running an LED at nonrated current, not to mention the 30k hour lifetime goes out the window, but this is an extreme eg. If you are talking about about a mere 10% over current on a small project however, you can expect things to pretty much behave as manufacturer intended.

In my experience a 700mA LED will take 1200 easily, with about 60% more light. (IMPORTANT: the 9 LEDs will use 46W instead of 27W if you used your current driver. If it cannot support this power, current will fall below and regulation will stop) lifetime sof the LEDs hould halve. If it is good brand, lifetime means gradual fading over time until it is more efficient to change the whole LED. Poorer brands will just die and open circuit. If you want to extend life, go for better cooling rather than undercurrent, because straying too far from rated values will cause the color temperature to shift also.
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Old 01-12-2014, 09:27 PM   #11
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Thanks 32103940, but even more unsure now as i have had a couple of conflicting bits of info. don't want to go wasting more money

Hoppy? As resident guru on all things led whats your take?
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Old 01-12-2014, 09:52 PM   #12
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Ok so decided to stop dithering and just purchased this 350ma 9-48v meanwell driver after rereading the DIY LED sticky

www.meanwell.com/search/lpc-20/LPC-20-spec.pdf‎


so in my ignorance. what does the wattage of the driver dictate? the rated watts for this driver 16.8W?
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Old 01-12-2014, 10:37 PM   #13
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You are really better off to use the lpc-20-700 rather than the 350. Im assuming you are using the 3w cheap ebay emitters?

Also the lpc divers from meanwell need to have a cord end soldered on and should technically be housed in an electrical box.


There are things I look at to determine a suitable driver.

1. Does the driver supply the proper current for my leds

2. Does the combined voltage fit into the voltage range of the driver.

3. Is the combined watts for all the leds less than the rated maximum wattage of the driver.
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Old 01-12-2014, 11:31 PM   #14
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If you are using non-Cree LEDs, you probably shouldn't run them at max current. I did that and even with a decent heatsink they burned out. Cree constructs their LEDs to survive higher temp than other manufacturers and is more honest in their max current rating.

As far as the "resistor" discussion, that idea won't work. Don't try. Previous discussion about the dynamic nature of constant current drivers was correct. Even if you did it with a "constant voltage" driver, you would still have a problem with high heat in the resistor. The energy has to go somewhere and high wattage resistors are expensive.
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Old 01-13-2014, 01:20 AM   #15
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Originally Posted by zimdoc View Post
Ok so decided to stop dithering and just purchased this 350ma 9-48v meanwell driver after rereading the DIY LED sticky

www.meanwell.com/search/lpc-20/LPC-20-spec.pdf‎


so in my ignorance. what does the wattage of the driver dictate? the rated watts for this driver 16.8W?
The rated wattage is the maximum power the driver is designed to handle. But, the LED's forward voltages added together and multiplied by the constant current the driver provides determines the actual power supplied by the LED driver. Divide that by the driver efficiency (at that voltage) to get the power the whole circuit consumes.

I consider O2surplus to be the reigning expert on driving LEDs.
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